1. Field
This disclosure is generally related to the fabrication of solar cells. More specifically, this disclosure is related to module fabrication of bifacial tunneling junction solar cells.
2. Related Art
The negative environmental impact of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photovoltaic effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal band gaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple single-junction structures of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
FIG. 1 presents a diagram illustrating an exemplary solar cell (prior art). Solar cell 100 includes an n-type doped Si substrate 102, a p+ silicon emitter layer 104, a front electrode grid 106, and an Al back electrode 108. Arrows in FIG. 1 indicate incident sunlight. As one can see from FIG. 1, Al back electrode 108 covers the entire backside of solar cell 100, hence preventing light absorption at the backside. Moreover, front electrode grid 106 often includes a metal grid that is opaque to sunlight, and casts a shadow on the front surface of solar cell 100. For a conventional solar cell, the front electrode grid can block up to 8% of the incident sunlight, thus significantly reducing the conversion efficiency.